The present invention relates to a new and improved capacitor having at least two electrodes separated from one another by an elastic dielectric formed of rubber and/or plastic for measuring forced acting upon the one electrode--the so-called measuring electrode--by detecting the resultant change in capacitance. The invention also concerns a method of measuring forces by means of such capacitor.
Especially in the case of large surface capacitor units there exists a basic problem which arises during the deformation of the elastic dielectric in terms of the limited transverse elongation possibility of rubber bodies which are clamped at a pressure surface and oppositely situated base surface. The forces acting upon the pressure surface are divided into two components, namely into a first component extending in the direction of deformation and a second component perpendicular thereto, in other words extending in a direction parallel to the electrodes. The force which extends in the direction of the electrodes causes transverse elongation of the elastic dielectric and, thus, affects the magnitude of the deformation region and thus the measuring region or range.
In German Pat. No. 1,916,496 of National Research Development Corporation there is taught to the art a capacitor wherein through the provision of hollow spaces or voids arranged in the electrodes, there is achieved a more favorable transverse elongation capability of the dielectric. Since in this case the hollow spaces in the electrodes serve for receiving the deformed material of the dielectric, the volume of such hollow spaces must be at least equal in size to the deformation arising during maximum loading of the capacitor, i.e. either the diameter of the hollow spaces must be very large with smaller thickness of the electrodes or else the electrode thickness must be large when the hole diameter is small. In the first-mentioned instance there, however, arises a weakening of the electrode plates as concerns the strength thereof, and in the last-mentioned case there is present too great rigidity which is unfavorable for an exact measurement result. Additionally, the force flow in the dielectric is extremely unfavorable, since the transverse elongation force effective in the electrode direction and derived from the force acting upon the capacitor must be further deflected at the region of the hollow spaces, and which force then extends in a direction opposite to the force which is to be measured.
This drawback can be somewhat alleviated by providing a nap-shape configuration of one of both electrode contact surfaces of the dielectric, as taught for instance in German Pat. No. 2,448,398 of Uniroyal Inc. Yet, when improving the flow of the forces in the dielectric there arise however drawbacks in the stability of the dielectric in relation to shear forces acting upon the electrodes, i.e., each force which does not act exactly perpendicular to the electrode can only be inaccurately measured due to the losses converted into shear forces.
A particular problem especially as concerns the measurement of dynamic forces resides in the non-linearity of the pressure or compression deformation characterisitic of a rubber elastic or elastomeric dielectric at the starting region. Here the last part of the recovery occurs over a relatively long period of time, so that the accuracy in the measurement of short successively following forces decreases with increasing frequency. Force pulses of approximately the same magnitude and following one another rapidly in succession are only capable of being determined in the form of a uniform capacitance change brought about by a static load.